1. Field of the Invention
This invention relates to radiant energy communications and particularly to such communications through mediums where radiated energy is subject to multipath delay spread effects and doppler frequency-shift effects.
2. Description of the Prior Art
In radiant energy communications systems, transmitters and receivers are often linked by a multiplicity of energy propagation paths. For example, if electromagnetic energy is transmitted through the atmosphere to communicate information, a first portion of the energy might be reflected at the surface of a first ionospheric layer and then travel directly to the receiver. A second portion of this energy might pass through the first layer and be reflected at the surface of a second, higher layer and then continue on to the receiver. Other paths of propagation are also possible, including paths followed by energy subjected to multiple reflections between the upper and lower surfaces of a single layer.
Another example of radiant energy communications susceptible to multipath propagation is that of acoustic energy transmitted through water. In this medium, the reflective surfaces include the water surface, the bottom, and the boundaries between water layers of differing temperatures.
As a consequence of such multipath propagation, a multiplicity of arrivals of energy are received for each piece of information transmitted. Because the multiple arrivals for each piece of information become interspersed with each other, it is difficult to discriminate between a first arrival of new information and a later arrival of information already received.
Discrimination between multipath arrivals representing individual pieces of information is further complicated if either the transmitter or the receiver is in motion relative to the medium of propagation. Such motion causes continuous changes in the paths of propagation between the transmitter and the receiver and these changes can affect both the number of arrivals and the sequence in which they are received. These variations are particularly evident in underwater communications because of the often radical variations of the bottom.
One technique suggested by the prior art for discriminating between multiple arrivals is that of transmitting energy in the form of a series of modulated pulses representing the information, the nominal frequencies of successive pulses being repeatedly shifted through a predetermined sequence of frequencies. This technique should enable discrimination between arrival pulses received at the receiver because multiple arrivals of each transmitted pulse will be received at the same distinct frequency. In practice, however, prior art attempts to reproduce the transmitted information from received arrivals have met with only limited success. This lack of success stems largely from a failure to discriminate between noise energy and energy originating from transmitted pulses.
One prior art adaptation of the frequency shifting technique attempts to discriminate against noise by utilizing the first arrival pulse received at each of the transmitted frequencies as the arrival pulse representative of the piece of information transmitted at that frequency. This adaptation is based on the premise that the first arrival pulse has a greater magnitude than later arrival pulses, because it travelled the shortest path, and has a greater magnitude than noise energy received. This is not realistic, however, because of magnitudes of the individual arrival pulses originating from each transmitted pulse depend not only on the distance of the path travelled but also on the reflectivity and absorptivity characteristics of the path. Nor does any particular path consistently produce the arrivals of greatest magnitude, because path characteristics continually change. These changes result both from gradual physical changes in the layers of the medium of propagation and also from motion of the transmitter or receiver relative to the medium. Thus, utilization of the first arrivals to discriminate againt noise is effective.
Another reason for the limited success of prior art adaptations of the frequency shifting technique is the failure of such adaptations to adequately compensate for doppler shifts of the transmitter frequencies when there is relative motion between the transmitter and the receiver. Broadening of receiver bandwidth to accommodate positive and negative doppler shifts of the transmitted frequencies is unacceptable because it increases receiver sensitivity to noise.